In eukaryotic organisms, a paradigm is emerging in which RNA functions as non-cell-autonomous signaling molecules (1–4). Full citations for the references are provided before the claims. In plants, a role for non-cell-autonomous RNA has been established in terms of systemic signaling associated both with RNA interference (RNAi) (1, 5–7) and development (8–10). Plasmodesmata (PD), the intercellular organelles of the plant kingdom (11), serve as the conduit through which proteins and RNA-protein complexes move, cell to cell, to exert supracellular control (11–15). The vascular system, and specifically the specialized cell-types of the phloem, provides the pathway for the long-distance translocation of non-cell-autonomous RNA-protein complexes, to distantly located tissues and organs (1, 5, 8–10). Delivery of such informational macromolecules into and out of the phloem translocation stream appears to occur through PD (10, 16).
The protein machinery involved in RNAi is under intense investigation (17). It is now evident that an RNase-III type enzyme, termed Dicer (18), and plant homologs of Dicer are pivotal to this process. Dicer binds and cleaves double-stranded (ds)RNA into 21–25 nt dsRNA species (19). These small RNA cleavage products then function as sequence-specific small interfering RNA (siRNA) or micro-RNA (miRNA) to mediate changes in gene expression by diverse mechanisms, including targeted degradation of gene transcripts, methylation and translational control (20, 21, 32). The cell-to-cell and systemic spread of RNAi is considered to occur through PD (3, 6) and the phloem (1, 3, 5–7), respectively; however, the RNA species and underlying mechanism of trafficking remain to be elucidated (7). There is thus a tremendous need both to better understand the underlying mechanism of RNA trafficking and to apply this knowledge to plant modification and improvement